Admin API to join users to a room. (#7051)

docs/admin_api/room_membership.md

@@ -0,0 +1,34 @@
+# Edit Room Membership API
+
+This API allows an administrator to join an user account with a given `user_id`
+to a room with a given `room_id_or_alias`. You can only modify the membership of
+local users. The server administrator must be in the room and have permission to
+invite users.
+
+## Parameters
+
+The following parameters are available:
+
+* `user_id` - Fully qualified user: for example, `@user:server.com`.
+* `room_id_or_alias` - The room identifier or alias to join: for example,
+  `!636q39766251:server.com`.
+
+## Usage
+
+```
+POST /_synapse/admin/v1/join/<room_id_or_alias>
+
+{
+  "user_id": "@user:server.com"
+}
+```
+
+Including an `access_token` of a server admin.
+
+Response:
+
+```
+{
+  "room_id": "!636q39766251:server.com"
+}
+```